The brain and the spinal cord are surrounded by cerebrospinal fluid (CSF), the primary purpose of which is to function as a support: the brain of a grown-up person weighs about 1.5 kg—placed in water the weight is about 50 g. CSF is formed in the cavities of the brain, the so called ventricles. From here, CSF is flowing through a passage system and out over the surface of the brain and spinal cord. Drainage occurs through special structures, so called arachnoidal villi, in connection with the venous blood vessels. The CSF system forms a fluid system with patient-dependent hydrodynamic properties.
There are several diseases which can affect the CSF system and a reduced drainage of CSF or an increased pressure inside the cranial cavity (intracranial pressure, ICP) can give rise to a number of symptoms. Hydrocephalus means that the ventricles of the brain increase in size and thereby, the amount of fluid therein. The symptoms of the patient can vary, sometimes ICP increases and the patient is suffering from headache and reduced wakefulness. A more common variant, the so called adult hydrocephalus syndrome (AHS) or normal pressure hydrocephalus (NPH), means that the patient suffers from a triade of symptoms with walking disorders, memory disorders and urinary incontinence.
Hydrocephalus can be treated with a CSF shunt. Every year, about 70000 shunts are implanted in the west, which makes it one of the most common neurosurgical operations. Therefore, it is important to find selection methods where the probability for a successful result is high. Such a selection method is built, inter alia, on determining the hydrodynamic properties of the CSF system of the patient.
The base for determining the hydrodynamic properties of the CSF system is built on insertion of needles, lumbarly or cranially, such that contact with the fluid system is obtained. Then, the intracranial pressure (ICP) is measured during an active infusion of artificial CSF. The connection between the infusion flow and ICP, as a function of time, is analyzed by means of a physiological model which may include system-defining parameters such as CSF production, outflow resistance, compliance, venous pressure etc. Thus, from the measurement, the patient values of these parameters are estimated. It is particularly the outflow resistance which today is used for determining the hydrodynamic properties of the CSF system.
A method commonly used today on many hospitals, makes use of an infusion pump with constant infusion rate which through a needle is connected lumbarly to the spinal cord canal. A resting pressure prior to start of the pump, a dynamic process with a pressure increase during 5 to 10 minutes after pump start and a sequence with equilibrium pressure during infusion are registered. The outflow resistance is determined based on the two equilibrium levels and compliance from the process of increase. Drawbacks with the method are that the precision of a determination based on two points as well as determination of a dynamic parameter in such a short time as 5 to 10 minutes is low and no statistic uncertainties are recorded. The method is manual and has no security connections between pump and pressure measurement and is therefore regarded as technically difficult to carry through as well as for final analysis.
A prior prototype of the present method and device was based on a feed-back system with two lumbarly located standard needles. ICP is registered through one needle and then, the flow is guided through the other needle by a control system the object of which is to keep the pressure on constant levels. The resting pressure is first determined and then, three to five different constant pressure levels are set and the flow for each level determined. Linear regression between the pressure and flow plots is used for determining the outflow resistance. This method is considered more accurate than the first, but has also no uncertainty analysis, utilizes a manual protocol and is technically demanding with advanced pressure and flow calibrations and manual final analysis. This method further requires a specially trained operator capable of determining by visual estimation when one shall go from one pressure level to another in the protocol.
A problem in connection with determining the hydrodynamic parameters of the CSF system is the extensive natural physiologic fluctuations of the intracranial pressure and the volume of the system. The fluctuations depend particularly on rhythmic fluctuations of the blood volume in the brain related to the heart rhythm and the breathing rhythm etc. These fluctuations might render it necessary to measure very small outflow volumes in an environment with extensive inner volume fluctuations (very low signal/noise relation). The physiologic fluctuations differ extensively from patient to patient. The present methods for measuring the CSF dynamics disregard variations between patients.